(57) Digital input commands defined in a page description language are separated in two
types of instructions : first instructions resulting in solid regions on the printed
output and second instructions resulting in halftoned regions. The first instructions
generate a binary bitmap indicating a high or low density of the solid regions and
a binary bitmask indicating whether recorder elements belong to a solid or screened
region. The bitmap and bitmask are preferentially compressed by a lossless compression
method. The second type of instructions generate a contone map, representing images
having contone levels and graphics having intermediate levels. The contone map is
preferentially compressed by a lossy compression method, stored on a storage medium
and retrieved once the output device must render the rasterized image. The rasterized
image is constructed on the fly from the compressed bitmap, bitmask and contone map,
which is halftoned before combination with the bitmap. A serious amount of memory
can be saved, while keeping up with the speed of the rendering process in the output
devise and without deteriorating the quality of the reproduction.
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